Vapor-compression refrigeration circuits for circulating a working fluid refrigerant having a low side comprising an evaporator and a high side comprising a compressor and condenser are well-known.
Refrigerant in a liquid state is metered to the low side of the refrigeration circuit by a metering control throttle valve. As the refrigerant flows through the low side evaporator, evaporator outlet line and other low side lines to the compressor, the refrigerant absorbs heat energy from the ambient environment. Heat energy absorbed by the refrigerant allows cooling of a desired space. As the refrigerant absorbs heat energy, it transforms from a liquid to a saturated liquid/vapor and finally to a superheated vapor state.
Superheated refrigerant vapor enters the high side of the refrigeration circuit at the compressor. The compressor compresses the vapor refrigerant into a high-pressure, superheated vapor, whereupon the refrigerant is flowed to the condenser. The condenser allows the refrigerant to reject heat energy to an ambient heat sink whereupon the refrigerant reverts to a liquid and is flowed back to the control valve and evaporator. The process is repeated through the refrigeration circuit to meet desired cooling loads.
Refrigeration circuits are designed to allow refrigerant to reach a set degree of superheat before refrigerant enters the compressor from the low side. This is done because at 0 degrees of superheat, the refrigerant is in a saturated liquid/vapor state as a mixture of saturated vapor and liquid. Compressors are designed to compress only refrigerant gas and can be damaged if liquid refrigerant enters the compressor. Often, refrigeration circuits are designed to allow refrigerant to reach a minimum set degree of superheat of about 10° F. is used to assure that the saturated liquid/vapor refrigerant has fully converted into a vapor before compression.
While configuring a refrigeration circuit to generate a set degree of superheat in refrigerant may be sufficient in steady-state cooling conditions, when evaporators are placed under variable cooling load conditions, operation errors can occur. Variable cooling load conditions cause rapid refrigerant vaporization and overfeeding conditions that require large superheat settings to assure all refrigerant is in a vapor state before exiting the evaporator. This necessitates setting a superheat setting. For instance, in DX evaporator industrial refrigeration applications, a superheat of 20° F. or higher may be used.
Large superheat settings significantly reduce refrigeration circuit efficiency and require refrigeration circuits to flow large quantities of refrigerant. A common refrigerant used in direct expansion evaporator applications is ammonia (NH3). While ammonia provides desired heat transferring properties, its use in large scale refrigerant applications presents risks including toxic inhalation, fire and explosion if there are refrigeration circuit or storage tank leaks.
Evaluating refrigerant status through the low side of the refrigeration circuit is important in developing efficient refrigeration circuits.
It is known to evaluate low side refrigerant status by placing a superheat sensor at the evaporator outlet line. The superheat sensor includes a temperature sensor and vapor pressure sensor to calculate the degree of superheat in vaporized refrigerant flowing by the sensor. If the detected amount of superheat in the fluid flow does not correspond with a set minimum desired superheat value, an alarm signal is sent to an electronic control system. The control system will then actuate the control throttle valve to reduce the flow of refrigerant entering the circuit low side.
It is also known to evaluate low side refrigerant status by placing a capacitive sensor at the evaporator outlet line. Capacitive sensors are commonly known as void fraction or quality sensors and provide a direct reading of the actual ratio of liquid present in a refrigeration circuit.
As void fraction sensors can only detect quantities of liquid refrigerant present within the circuit they are unable to detect refrigerant superheat. A void fraction sensor's output is directly proportional to the percentage of measured liquid state refrigerant flowing through the sensor. In such systems, the refrigeration circuit is configured so that refrigerant flowing from evaporator and through the evaporator outlet line is in a saturated liquid/vapor state containing a low percentage of liquid state refrigerant. If the detected amount of liquid refrigerant is above a set maximum desired value, an alarm signal is sent to an electronic control system. The control system will then actuate the control throttle valve to reduce the flow of refrigerant entering the circuit low side.
Such a system is described in U.S. Patent Publication number US20130291568 A1. The publication further describes a refrigeration circuit including a second void fraction sensor immediately upstream of the compressor. The second void fraction sensor acts as a safety for the compressor, sending a shutdown or speed reduction signal to the compressor when liquid state refrigerant is detected.
A limitation with evaluating low side refrigerant status with a void fraction sensor is that in any case of a refrigerant being at 0 degrees of superheat or above, the void fraction sensor's output remains null. This leads to potential system inefficiently and malfunction, especially in changing transient cooling load conditions.
Another limitation with the above described refrigeration circuit control systems is that the superheat and void fraction sensors provide control feedback based on set superheat or liquid state refrigerant percentage values at the exit of an evaporator. In variable cooling load conditions, feedback from a single sensor too slow to allow the system to properly meter control valve refrigerant flow to the low side. This problem is heightened by sensors being installed downstream from the system evaporator. The evaporator provides the highest rate of heat exchange for the low side of the refrigeration circuit and may contain hundreds of feet of line through which a metered working fluid must flow before the sensor is reached and a change in system status is detected. By the time the void-fraction sensor provides an indication to the control system that refrigerant liquid content is too high or too low and corresponding valve actuation signals can be sent to the far upstream metering valve, it is too late to provide appropriate control of evaporator-downstream refrigerant vapor content.
In the case of too little refrigerant flow, this results in premature refrigerant over boiling within the evaporator, which decreases evaporator efficiently. In the case of too much refrigerant flow, this allows liquid refrigerant to exit the evaporator, necessitating additional system components such as liquid traps to protect the compressor. This adds to system installation costs and likewise lowers system efficiency.
Therefore there is a need for a refrigeration circuit control system that detects refrigerant state conditions and controls the circuit to increase efficiently and to reduce the quantity of working fluid needed to meet cooling load conditions, especially under transient cooling loads.